With development and advancement of technology, efficiency and convenience are important orientations in the use of electronic products that are desirably made with low profile, multiple functions and highly efficient operation. In respect of semiconductor industry and integrated circuit (IC) design, although it has successfully attained to important improvements such as profile miniaturization, high integration and multi-functions for electronic elements, however, a reliability issue is generated due to heat production during operation of the electronic elements. In particular, as power-to-work conversion is of a rate not possibly achieving 100%, it means a portion of power is not consumed by operation of the electronic elements but becomes heat that may significantly raise temperature of the entire operating system of electronic elements. If the operating temperature is raised above an upper limit set for safe operation, the system may break down or become failure by virtue of over heat. For advanced new-generation electronic products, internal electronic elements thereof are arranged in higher density and operate at a higher speed than traditional electronic products, thereby producing relatively more heat during operation of the advanced electronic products and easily over raising the operating temperature. For example of a central processing unit (CPU) used in a personal computer (PC) and manufactured by KryoTech Company in U.S.A., a heat production rate of the CPU has increased from 40W of year 1996 to 100W of year 2000; however, a common cooling device having a heat sink and a small fan mounted in association with the CPU can only dissipate 60W of heat. Therefore, higher efficient heat dissipating technology such as fluid cooling or phase variation cooling is greatly required. Moreover, in compliance with low profile electronic elements, associated heat dissipating devices are preferably made with compact size and low weight to be integrated into the electronic elements, which would render a challenge for heat dissipating technology.
A current solution to the foregoing heat dissipating problem is the use of gradually matured MEMS (micro electrical mechanical system) technology that can produce an advanced micro heat dissipating device by relevant semiconductor fabrication processes. As shown in FIG. 5 of a micro heat exchanger 3 with a plurality of micro channels 35, it is formed by subjecting silicon substrates to fabrication processes including film deposition, photo-lithography, wet etching and micro packaging. In particular, the silicon substrates are each etched to form a plurality of parallel micro grooves and then attached to each other to form a plurality of micro channels 35 through which a cooling fluid 5 passes; alternatively, the silicon substrates are stacked to form a heat exchanger with multiple layers of micro channels 35. One surface of the silicon substrate can be mounted with a heat generation source such as IC chip, allowing heat produced from the chip to be directly taken away via the cooling fluid. As the silicon substrate has higher coefficient of heat conduction than a common substrate and similar coefficient of thermal expansion to a silicon chip, it can thereby prevent the chip from being damaged by thermal stress in response to increase in temperature, and also helps enhance heat dissipating efficiency of the micro heat exchanger 3.
However, the above conventional micro heat exchanger 3 still has drawbacks in association with a normal fluid cooling device, that is, the cooling fluid 5 undesirably becomes a source of heat resistance. For the micro channels 35 of the micro heat exchanger 3, the cooling fluid 5 is of a motion with low Reynolds number and may not achieve good heat dissipating effect. In view of a single micro channel 35, due to fluid cohesion, a portion of fluid close to an inner wall of the micro channel 35 may produce a boundary layer 60 with velocity of zero adjacent to the inner wall 35′ of the micro channel 35 as shown in FIG. 6; this makes the fluid 5 not flow uniformly in the micro channel 35 in a manner that, mass flow rates at different positions in the micro channel 35 are identical, but flowing and mixing of the fluid 5 are not uniformly formed. Moreover, by heat exchange between the fluid 5 and the inner wall 35′, a temperature gradient of the fluid 5 is produced along the inner wall 35′ to form a thermal boundary layer that affects heat transfer performance of the heat exchanger 3. Furthermore, besides effect of boundary layer in each of the micro channels 35, non-uniform fluid flows in different micro channels 35 would lead to reduction in thermal transmission. In view of a formula for mass flow rate:mass flow rate=ρ×A×vwherein ρ, A, v respectively represent fluid density, cross-sectional area and velocity, for different micro channels 35 located on the same plane, different velocitys v and fluid densities ρ in the micro channels 35 lead to different mass flow rates of fluids 5 in different channels 35. Therefore, as shown in FIGS. 7A, 7B and 7C, it may be the case that central channels 35a or side channels 35b have relatively higher mass flow rates, or mass flow rates in different channels 35 are not regularly distributed, depending on channel size and resistance, location of the heat exchanger, fluid velocity, fluid type, temperature, etc. This irregular distribution of mass flow rates would undesirably reduce heat transfer performance and heat dissipating efficiency of the heat exchanger.
Therefore, the problem to be solved herein is to provide an enhanced heat transfer device, which can measure a mass flow rate of fluid in each micro channel and facilitate uniform fluid flows in different channels in a manner as to minimize effect of thermal boundary layer of fluid in each channel and to improve heat transfer performance of a micro heat exchanger.